Video-on-demand systems allow subscribers to request video programs from a video library at any time for immediate viewing in their homes. Subscribers submit requests to a video service provider via a communication channel (e.g., telephone lines or a back channel through the distribution network that carries the video to the subscriber's home), and the requested video program is routed to the subscriber's home via telephone or coaxial television lines. In order to provide such movie-on-demand services, video service providers use a video server to process subscriber requests, retrieve the requested programs from storage, and distribute the programs to the appropriate subscriber(s). One exemplary system for providing video-on-demand services is described in commonly assigned U.S. Pat. No. 6,253,375, issued Jun. 26, 2001, which is incorporated herein by reference.
In order for video servers to provide good performance, it is crucial to schedule video storage (disk) access requests such that disk bandwidth is maximized. Also, once a subscriber is watching a program, it is imperative to continuously deliver program content to the subscriber without interruption. In addition to distributing content to subscribers, disk bandwidth in a video server is typically also utilized for operations such as loading content, disk maintenance, and file system meta-data synchronizing. Disk bandwidth may also be reserved for reducing latency in data transfer to subscribers. The number of subscribers that can be properly served concurrently by a video server therefore depends on effective disk bandwidth, which in turn depends on how disk access requests are scheduled.
One of the problems facing current disk scheduling methods is the potential variation in time required to service disk accesses. For example, the internal transfer rate of a SEAGATE CHEETAH™ disk varies from 152 Megabits per second (Mbps) on inner tracks to 231 Mbps on outer tracks, and the seek time can vary from 0 milliseconds (ms) to 13 ms depending on how far apart the segments of data are from one another. Given these variations in seek and transfer times and the fact that the server may contain sixteen or more disk drives, it is difficult to determine the effective disk bandwidth of a video server. As a result, current disk scheduling methods allocate a fixed amount of time for every disk access request, regardless of whether the access finishes early. This results in a deterministic system in which the available disk bandwidth is known, but since the fixed amount of time must be large enough to accommodate a worst-case disk access, disk bandwidth is wasted.
Therefore, there is a need in the art for a method and apparatus for scheduling disk access requests in a video server without allocating worst-case access times, thus improving disk bandwidth utilization.